Clusters of galaxies are dense, extreme environments. Here, galaxies lose their gas, stars, and dark matter through a variety of physical processes. These massive regions have deep potential wells, where galaxies can be tidally stripped of weakly bound material, and galaxies can interact and collide with other galaxies. But the really interesting physics is in what the gas does.

(I am happy to share any of these animations to use in classes, lectures, seminars, etc., with proper attribution! Feel free to download them on Vimeo, or email me at rukmani[at]virginia.edu)

Ram Pressure Stripping

Galaxy clusters are filled with hot, X-ray emitting gas at tens of millions of degrees, called the intracluster medium (ICM). As galaxies orbit through the ICM, they experience a drag force, otherwise referred to as 'ram pressure'. This ram pressure can remove gas bound to galaxies. Ram pressure is proportional to the local density of the gas times the velocity squared of the galaxy: so the faster the galaxy moves (which in turn depends on how massive the cluster is -- galaxies in massive systems move faster), the more ram pressure is experiences and loses its gas. This process, called ram pressure stripping, forms long tails of gas that trail galaxies in their orbits. We see these ram pressure stripped tails in cluster galaxies, for example ESO 137-001, M86, NGC 4402, and more.

Here are two numerical simulations where we have a galaxy group and a galaxy cluster, initially full of gas-rich galaxies. As galaxies orbit within the cluster, they lose their gas, form spectacular tails which dissolve in the ICM, and finally lose their central bound coronae. These simulations are published in Vijayaraghavan & Ricker (2015).

magnetic fields

The ICM is threaded with weak (~micro Gauss strength) magnetic fields. These magnetic fields add a layer of complexity to understanding how galaxies are stripped of their gas. When galaxies move through the magnetized ICM, magnetic field lines are pushed together at the galaxy-ICM interface and wrap around the galaxy, a phenomenon called 'magnetic draping'. A few interesting things happen -- these draped magnetic fields can suppress Kelvin-Helmholtz, or shear instabilities (seen in the previous movies) at the ICM-galaxy interface, and orbiting galaxies themselves stir up and amplify ICM magnetic fields.

In these animations of a group and cluster, slices of density (left) show what happens to the gas as galaxies orbit: galaxy gas is gradually stripped, forms tails, and dissipates. The slices of the plasma beta parameter (right), which is inversely proportional to the magnetic field strength, show that magnetic fields are amplified as galaxies pass through them. Arrows correspond to the magnetic field vector.

Thermal conduction

A physical process that can be even more efficient at removing gas from galaxies than ram pressure is the evaporation of gas from thermal conduction. The ICM is an ionized plasma, and ICM electrons can efficiently transport heat from the hot ICM to the cooler ISM gas, particularly the warm X-ray emitting ISM.

This animation on the right shows a slice of a galaxy in a 'wind-tunnel' -- a simulation box in the reference frame of a galaxy embedded in an ICM-like gas. The ICM flows past the galaxy at typical galaxy speeds in clusters -- here, at 600 km/s. When thermal conduction is not included, and the galaxy does not evaporate, it loses its gas from ram pressure stripping in approximately two billion years.

When thermal conduction is turned on for the same galaxy under identical ICM conditions, and heat is allowed to efficiently flow from hotter ICM to the relatively cool galaxy, the galaxy rapidly evaporates by 150 million years. Evaporation from isotropic, saturated thermal conduction is about 10 to 20 times faster at removing gas than ram pressure stripping.

Of course, this is unlikely to be as effective in real clusters and galaxies. We see tails and long-lived X-ray emitting coronae from galaxies, indirectly implying that thermal conduction is simply not as efficient as a straightforward physical estimate would lead us to believe. Why is thermal conduction not as efficient? The answer most likely is because of magnetic fields, which result in anisotropic thermal conduction.

anisotropic thermal conduction

In the presence of magnetic fields, electrons rotate, or gyrate around field lines. The radius of gyration in typical ICM conditions is approximately 10,000 km, significantly smaller than the kpc-scale electron mean free paths in the ICM. As a result, electrons are trapped by magnetic fields and are largely allowed to only travel parallel to the magnetic field, rather than isotropically. Therefore, heat conduction is anisotropic, i.e, heat is allowed to flow from the ICM to the ISM only along magnetic field lines. This slows the rate at which gas can evaporate from galaxies, and restricts heat flow only to directions along the local magnetic field. Although heat conduction is slowed down, ram pressure is still effective at removing gas, but much slower than isotropic thermal conduction.

The animations here show what happens when anisotropic conduction acts on a magnetized ISM and ICM for the galaxy in a wind tunnel problem from the previous section. Over a range of extreme relative magnetic field orientations (parallel and perpendicular to the ICM wind), and morphologies (fully continuous and fully separate ICM and ISM fields), we see that heat flows are significantly slowed down. Another immediately apparently phenomenon is that heat flows are restricted to the direction of the magnetic field.